1. Field of the Invention
The invention relates to a method whereby stations may access a common transmission medium, and to a network station for implementing the method.
2. Description of the Related Art
In communication systems such as GSM (Global System for Mobile Communication), UMTS (Universal Mobile Telecommunication System), and HiperLAN (LAN: Local Area Network), mobile stations communicate with network-side stations over radio interfaces. The radio interfaces can here be employed by a plurality of mobile stations in parallel or concurrently. The accessing by stations of a common transmission medium of this type is facilitated by multiple access (MA) systems, whereby a distinction is made as to whether the medium is divided among the stations on a Time Division Multiple Access (TDMA) basis, a Frequency Division Multiple Access (FDMA) basis, a Code Division Multiple Access (CDMA) basis or a Space Division Multiple Access (SDMA) basis. The medium is frequently subdivided into frequency channels and/or time slots, such as in the case of GSM, TETRA (Trans-European Trunked Radio), and DECT (Digital European Cordless Telecommunications). Measurements are employed in the case of systems coordinated on a decentralized basis to determine whether a channel can be used. In keeping with the radio propagation it is possible to re-use a channel, for instance at an appropriate spatial distance.
Future radio systems should be able to support a large number of different services simultaneously. The services differ in having, for example, different transmission requirements in terms of delay or error rate, for instance, and arrival processes. The aim of the carrier is to maintain the quality-of-service agreed during connection setup and to use the available spectrum as efficiently as possible.
An occurrence in communication systems coordinated on a decentralized basis whose channel assignment is based on radio channel measurements is for a station to require transmission capacity for a connection but for this transmission capacity to be unavailable, the neighboring stations having occupied this capacity or the relevant channel. As a consequence, either the connect request has to be rejected or the agreed quality-of-service of existing connections cannot be maintained. In communication systems with link adaptation, such as EDGE/EGPRS (Enhanced Data Rates for GSM Evolution/Enhanced General Packet Radio Service), the occupation of a channel by a neighboring station may necessitate a change of modulation/coding, thereby reducing the available data rate and consequently negatively affecting existing connections.
A particular occurrence when connections with different quality-of-service requirements are operated simultaneously is for data services with low-level requirements, such as file transfers (FTP/File Transfer Protocol downloading), to require and occupy more transmission capacity during connection setup than is necessary to maintain the quality-of-service required by the user if this is available at that time, for example if a file transfer is being performed. The relevant capacity will then cease to be available at the neighboring stations and connect requests will have to be rejected.
There is therefore the problem of distributing the existing transmission capacity among radio cells organized on a decentralized basis as efficiently as possible, whereby, on the one hand, the capacity must be utilized as fully as possible even if this results in exceeding the required quality-of-service, and, on the other hand, any non-essential capacity can, if necessary, be made available to other radio cells.
This problem arises in systems in which services with different quality-of-service requirements share the common radio medium and in which use is made of measuring, dynamic channel assignment organized on a decentralized basis. The specific type of channel, such as time slot and/or frequency channel, is basically not of crucial significance here. There are currently various systems of this type:
In the IEEE 802.11 standard there is a mode controlled on a centralized and a decentralized basis. In the case of decentralized control (Distributed Coordination Function (DCF)), all stations are peer entities and service prioritizing is not supported. In the case of centralized control (Point Coordination Function (PCF)), accessing by the individual stations in a radio cell is controlled by a central station serving as the access point (AP), with several access points being connected via a distributed system (DS). The problem described will not be resolved if the access points are unable to communicate with each other, for example if the central controller (PCF) is used by a station without access to a distribution system (DS) in order to support a specific quality-of-service in an ad hoc mode. With decentralized access, the transmission of a single packet could be controlled by a priority specified according to, among other things, the requirements of the quality-of-service. However, this is currently not supported by the standard.
In the HIPERLAN/2 system for data networks currently being standardized, a base station (Access Point (AP)) manages the access to the common medium by the cordless stations (Wireless Terminal, WT) registered with it. Several base stations can be coordinated by a higher-order instance (Access Point Controller (APC)). The standard only allows for central administration of the transmission capacity within the area of an access point. If an access point does not have a connection to the fixed network, the access point is referred to as a central controller (CC).